Appendix A. Regular Expressions

A. Regular Expressions

IN THIS APPENDIX

Characters	968
Delimiters	968
Simple Strings	968
Special Characters	968
Rules	971
Bracketing Expressions	972
The Replacement String	972
Extended Regular Expressions	973

A regular expression defines a set of one or more strings of characters. A simple string of characters is a regular expression that defines one string of characters: itself. A more complex regular expression uses letters, numbers, and special characters to define many different strings of characters. A regular expression is said to match any string it defines.

This appendix describes the regular expressions used by ed, vim, emacs, grep, gawk, sed, and other utilities. The regular expressions used in shell ambiguous file references are different and are described in "Filename Generation/Pathname Expansion" on page 221.